In a dynamic random access memory (DRAM), in order to shorten a test time and guarantee the operation of a device over several years, a burn-in test is carried out by applying abrupt stress to the device. Such conventional burn-in tests have employed schemes for screening defects in advance by applying stress mainly to Oxide-Nitride-Oxide (ONO) films, cells and bit lines. These screening schemes may include, for example, a scheme for applying stress to an ONO film by applying a high voltage to a word line, writing high-voltage data into a cell using a bit line precharge voltage VBLP and then grounding a VCP voltage, and a scheme for applying stress between adjacent cells or adjacent bit lines by applying voltages of different polarities to the adjacent cells or adjacent bit lines to impress a high voltage therebetween.
Referring to FIG. 15, a semiconductor device comprises an output switch 150 including n-channel metal oxide semiconductor (NMOS) transistors N159 and N160 connected respectively between bit lines BL and BLB and segment input/output lines SIO and SIOB, for transferring data placed on the bit lines BL and BLB to the segment input/output lines SIO and SIOB in response to an output enable signal Yi. The output switch 150 has a single contact because it is small in layout area. However, if at least one of the contacts of such output switches has an initial defect, the quality of a product and the package yield thereof will be degraded, thus adversely affecting reliability of the device. Therefore, there is a need to screen a contact defect of the output switch 150 in advance.